The two major pathological hallmarks of Alzheimer's disease are the neurofibrillary tangles and the senile plaques. Senile plaques consist of extracellular amyloid fibrils, composed of the beta-amyloid peptide, a proteolytic fragment of the beta-amyloid precursor protein surrounded by dystrophic neurites, activated microglia and astrocytes. Other proteins are also found in the AD plaques, including alpha1-antichymotrypsin and apolipoprotein E. The neurofibrillary tangles in neuronal cell bodies are composed of paired helical filaments (PHF). A large number of studies have now shown that PHFs are made up of the microtubule associated protein tau, which is abnormally phosphorylated. Recent studies have shown that lys-ser- pro (KSP) sequence on tau are among the sequences abnormally phosphorylated in Alzheimer's Disease. The high molecular weight neurofilament protein, NF-H is phosphorylated on similar KSP consensus sequences. This sequence is present more than fifty times in the tail of the NF-H molecule and most of these sites are normally phosphorylated in vivo. Recent studies have shown that cdk5, a kinase related to the cell cycle dependent kinase cdc2, is expressed in the brain and associates with neurofilaments, as well as microtubules. This kinase phosphorylates some, but not all of the KSP sites of NF-H and is also able to phosphorylate tau on some of the sites abnormally phosphorylated in AD. This proposal focuses on the specific function of this kinase in the nervous system and how it may relate to the abnormal phosphorylation of tau in AD. In addition, we will attempt to isolate other kinases which phosphorylate NF-H on the remaining KSP sites, and which may also act abnormally on tau in Alzheimer's Disease. The aims of this proposal are: 1. To study the effects of overexpression of cdk5 on the phosphorylation of NFH and tau in vivo by introducing its cDNAs cloned in a neuronal expression vector in transgenic mice. 2. To determine the effect of inhibition of cdk5 in transgenic mice by mutating the cdk5 cDNA clone in its active site t produce an inactive kinase, which will inhibit the endogenous ckd5. This mutant kinase will be introduced into transgenic mice with a neuron specific expression vector and we will determine the effect of the inhibition of phosphorylation of NF-H and tau. 3. To isolate other kinases which phosphorylate NF-H on the remaining KSP sites by protein chemical methods, as well as the yeast two-hybrid system.